Electronic processing of graphic and text images produces multi-color prints using multiple color separations. Typically, four process colors, cyan, magenta, yellow and black, are used to print multiple separations, which tend to have minor misregistration problems. The result of abutting or overlapping shapes is a boundary between adjacent regions of color that, under ideal printing conditions should have zero width. That is, one color should stop exactly where the abutting color begins, with no new colors being introduced along the boundary by the printing process itself. The “colors” which fill the shapes can be solid colors, tints, degrades, contone images, or “no fill” (i.e., the paper with no ink applied). In general, the “colors” represented in these adjacent regions are printed using more than one colorant. In practice therefore, the realization of a zero width boundary between regions of different color is impossible as a result of small but visible misregistration problems from one printed separation to another. The error is manifested as a “light leak” or as a visible boundary region of an undesired color.
Methods for correcting for this misregistration are known. The general approach is to expand one of the abutting regions' separations to fill the gap or misregistration border region with a color determined to minimize the visual effect when printed. Borders or edges expanded from a region of one color to another in this manner are said to be “spread”. A border which has been expanded is referred to as a “trap”, and the zone within which color is added is called the “trap zone”. Edge detection and image manipulation to perform trapping may be done in any of several processes, including for example, the technique described in U.S. Pat. No. 6,345,117 to Victor Klassen, for “Method for Automatic Trap Selection for Correcting for Separation Misregistration in Color Printing”.
Certain marking systems produce variations in desired output color from print engine to print engine. These variations are due to specific physical characteristics of the particular print engine. A common technique for correcting for variations in color output is to measure a set of printed colors against a set of control colors and to provide a lookup table generated from an analytic function (such as a gamma correction function) to correct for variations in color output. Thus, all print engines of a particular model can be corrected to have the same desired output color.
Certain marking systems exhibit difficulty maintaining color uniformity near edges. For example, it may be intended to place a 75% fill, which may be possible in the body of the fill object; however, within a few millimeters of the edges, more or less than 75% may be deposited, depending on the marking process. For trapping, this can be a significant problem, as the trapping operation fundamentally only modifies edge pixels of abutting objects. If the trapping engine specifies a trap color (for a given separation) of 35%, only 20% may be produced as a result of the edge physics, for example. It is therefore desirable to have the capability to correct for these edge modulations, in order to achieve traps that are more pleasing to the eye.